Printing is one of the most popular ways of conveying information to members of the general public. Digital printing using raster printers allows rapid printing of text and graphics stored on computing devices such as personal computers. These printing methods allow rapid conversion of ideas and concepts to printed product at an economic price without time consuming and specialised production of intermediate printing plates such as lithographic plates. The development of digital printing methods has made printing an economic reality for the average person even in the home environment.
Conventional methods of raster printing often involve the use of a printhead, e.g. an ink jet printhead, with a plurality of marking elements, e.g. ink jet nozzles. The marking elements transfer a marking material, e.g. ink or resin, from the printhead to a printing medium, e.g. paper or plastic. The printing may be monochrome, e.g. black, or multi-coloured, e.g. full colour printing using a CMY (cyan, magenta, yellow, black=a process black made up of a combination of C, M, Y), a CMYK (cyan, magenta, yellow, black), or a specialised colour scheme (e.g. CMYK plus one or more additional spot or specialised colours). To make a print on a printing medium such as paper or plastic, the marking elements are “fired” in a specific order while the printing medium is moved relative to the marking elements. Each time a marking element is fired, marking material, e.g. ink, is transferred to the printing medium by a method depending on the printing technology used. Typically, in one form of printer, the head will be moved relative to the printing medium to produce a so-called raster line which extends in a first direction, e.g. across a page. The first direction is sometimes called the “fast scan” direction. A raster line comprises a series of dots delivered onto the printing medium by the marking elements of the printhead. The printing medium is moved, usually intermittently, in a second direction perpendicular to the first direction. The second direction is often called the “slow scan” direction.
The combination of moving the printhead relative to the printing medium while printing raster lines, and moving the printing medium relative to the printhead while not printing results in a series of parallel raster lines which are usually closely spaced. Seen from a distance, the human eye perceives a complete image and does not resolve the image into individual dots provided these dots are close enough together. Closely spaced dots of different colours are not distinguishable individually but give the impression of a blended colour determined by the amount or intensity of the different composing colours, e.g. cyan, magenta and yellow which have been applied.
In order to improve the image reproducibility of the printing method, e.g. of a straight line, it is preferred if the distance between dots of the raster is small, that is the printing has a high resolution. Although it cannot be said that high resolution always means good printing, it is true that a minimum resolution is necessary for high quality printing. A small dot spacing in the slow scan direction means a small distance between marker elements on the printhead, whereas regularly and small dot spacing in the fast scan direction places constraints on the quality of the drives used to move the printhead relative to the printing medium in the fast scan direction.
Generally, there are mechanisms for positioning a marker element in a proper location over the printing medium before it is fired. Usually, such drive mechanisms are controlled by a microprocessor, a programmable digital device such as a PAL, a PLA, an FPGA or similar although the skilled person will appreciate that anything controlled by software can also be controlled by dedicated hardware and that software is only one implementation strategy.
To be successful in the market, ink jet printing presses should combine good grey-scale capabilities, high printing speeds, and good reliability. This is not easily achievable with current prior art systems and concepts.
It is known from US-2002/0105557 to generate gradation levels or grey levels in a printed image by a combination of different sizes of printed dots. Two embodiments of printheads are described. According to a first embodiment, a printhead for a given colour comprises two head chips for that colour, and each head chip comprises a nozzle row with nozzles with different area, e.g. large and small nozzles, from which different amounts or volumes of ink are ejected. The large and small nozzles are alternately arranged in each of the nozzle rows. Furthermore, corresponding nozzles on each of the head chips, i.e. e.g. an xth nozzle on each of the head chips, have a different area, i.e. if the xth nozzle on the first head chip is a large one, the corresponding xth nozzle on the second head chip is a small one and vice versa.
According to a second embodiment, two head chips are provided each of which has only nozzles from which the larger or smaller ink droplet is ejected, i.e. a first head chip has all large nozzles and a second head chip has all small nozzles. The two head chips from which ink droplets of the same colour but different sizes are ejected have respective arrangements of nozzles which are offset from each other in a direction perpendicular to a scanning direction of the two head chips.
In both above embodiments, a pixel location on a printed medium is printed with either no droplet, a droplet of a small size or a droplet of a large size, different pixels together forming a superpixel having a grey level depending on the droplets actually printed. A superpixel, e.g. a 2×2 pixel matrix, using 2 levels of droplet sizes per pixelis thus built. In such 2×2 superpixels with two levels of droplet size, in theory nine distinguishable grey levels can be generated. With the above system, grey level images are printed with a resolution which is half of the resolution of the printheads, sets of two neighbouring nozzles of the head chips generating dots in one superpixel, each superpixel forming one imagepixel. The head chips for each colour ink are bonded to each other to from an integral printhead. 2×2 dot patterns disposing larger and smaller dots can be formed during a single scan operation. Obtaining grey levels this way is called dithering.
It is very difficult to make a printhead in which a marking element is suitable for printing droplets which differ in volume from each other to a large extent, e.g. a printhead marking element suitable for firing both droplets of 5 picoliter and droplets of 40 picoliter. If it is desired to make a printhead suitable for optimally printing such different droplets, such head becomes very expensive. It is a further disadvantage of the system of US-2002/0105557 that the image printed has a resolution which is only half of the resolution of the printhead. So improved grey scaling has been traded-off with resolution. Also, firing times of different drop sizes must be carefully controlled as the velocity of different drop sizes and delay before ejection of different drop sizes may be different. This is known as droplet ballistics. Due to the speed of a scanning ink jet printhead when traversing, any change in droplet velocity or delay time of ejection will result in the drops landing at a different place on the printing medium. Thus, if different drop sizes are used, the control mechanisms must be complex. For example as disclosed in U.S. Pat. No. 4,714,935 and EP 902 587, the real-time firing of each drop has to be controlled individually so that the droplets with different sizes hit the printing medium at the correct place.
For obtaining a lot of grey scale or contone levels, often a plurality of small droplets, e.g. 16, 29 or 32 small droplets, are combined to form a plurality of levels of ink load, e.g. 17, 30 or 33, see for example “Printer Handbook”, M. L. Chambers, IDG books, 2nd edition, 2000, especially chapter 3. The more ink is applied to the printing medium, the larger the size of the printed dot and the darker the image. This is called area modulated printing. However, this means that the printing device must be able to fire a small droplet of ink at a same pixel position on the printing medium a plurality of times, e.g. 16, 19 or 32 times. Such a printing device will be slower, e.g. 16, 19 or 32 times slower, than a binary printing device. Improved grey scaling has thus been traded-off with printing speed.
It is also known to do contone printing using time modulation. In that case more contone levels means reduction of the standard firing frequency, and thus also a slower printing speed.
There is a need for a method and a device for printing contone images at a speed which is higher than the speed of known contone printing devices, and with a better image quality.